This invention generally relates to suction roll apparatus used in papermaking, and more particularly, to an apparatus for improved removal of water from holes in a suction roll.
In paper production, water is removed from a wet web of paper pulp carried on a felt by passing the web through the nip of a pair of press rolls. A suction roll is commonly used as one of these press rolls for water removal from the paper web. A suction roll contains drilled or through-holes (and may also contain blind holes or grooves) for accommodating water expressed from the web while the web is in the nip. An internal suction box is provided in the suction roll near the nip area. The suction box is stationary and is aligned more or less with the nip contact area. Its purpose is to draw expressed water into the drilled holes. It is desirable that any residual water remaining in the holes after the holes rotate past the suction box be slung or thrown from the holes as the suction roll rotates away from the nip area. However, there is a tendency for some water and debris to remain in the holes, and for water sling or water throw to occur at an undesirable circumferential location, that is, for the residual water to be thrown back into the ingoing felt or web ahead of the nip area. This water xe2x80x9ccarryoverxe2x80x9d is detrimental to web dewatering in the nip, and can also cause non-uniform conditions to occur in the web or in the press felt.
In some cases, residual water in the drilled or through-holes may remain in the holes and not be thrown from the roll at any circumferential position. This is also undesirable, as the holes should be free of water and debris at the ingoing side of the nip if optimum water removal efficiency is to be achieved. If residual water remains in the holes, nip dewatering efficiency suffers.
In yet other cases, namely those situations where the suction roll is equipped with a doctor blade, residual water in the through-holes may be expelled only after the holes pass across the doctor blade. This post-blade dewatering results from a momentary negative pressure pulse exerted on the holes as the holes and doctor blade diverge. This too can be undesirable, as water expelled after the doctor blade may be directed towards the ingoing nip, thereby compromising nip dewatering efficiency.
By way of example, a suction press roll is used to describe the present invention. However, this invention applies also to any suction roll that is used for water removal on a paper machine. Such rolls include forming rolls, suction couch rolls, and suction press rolls. Furthermore, forming rolls and couch rolls may not be in contact with a second roll (i.e., there may not be a true nip formed with a second roll). For these rolls, the term xe2x80x9cnipxe2x80x9d is meant to imply the area adjacent to the suction box in the suction roll.
Conventional suction roll equipment for papermaking has not provided a solution to prevent water sling or carryover to the web. U.S. Pat. Nos. 5,466,342 and 5,466,343 to Kankaanpaa (Valmet) are of interest for disclosing an internal water jet for forcing water and debris out of holes in a suction roll into a water collecting trough. U.S. Pat. No. 4,693,784 to Aula (Valmet) is of interest for disclosing the use of air jets directed tangential to the inner roll surface away from the end zones of the suction area of a suction roll as air seals obviating the need for mechanical seals. U.S. Pat. No. 5,178,731 is also of interest showing the use of a doctor blade to create negative pressure on the roll cover surface, thereby providing a differential pressure in effect to suck water from the holes.
A principal object of the present invention is to provide a method and apparatus for dewatering through-holes in a suction press roll, suction couch roll, or forming roll to avoid water sling or water carryover back to the felt, wire, or web as the suction roll continues to rotate. A specific object of the invention is to provide for removing the residual water in the holes of a suction roll at a desired circumferential location before it can be carried back to the nip. Another objective is to provide a solution that can be applied to different types of suction rolls and used in the press section, at the couch, or as forming rolls.
In accordance with the invention, a method and apparatus for dewatering a suction roll in a paper machine has an air delivery device mounted in the inner volume of the suction roll for applying a radially outward force against the inner surface of the suction roll shell to expel residual water from the through-holes in the suction roll. The air delivery device is positioned at a suitable circumferential position relative to the suction box to prevent water carryover back to the nip, whether this water would otherwise be physically thrown from the holes to the ingoing felt, web, or wire, or whether this water would otherwise remain in the holes without being thrown. It may be conveniently mounted to the suction box assembly on the inside of the suction roll.
In a first embodiment of the invention, a blowbox is mounted in the inner volume of the suction roll at a circumferential position such that the expelled water will not be thrown back to the ingoing path of the web. It can be used as a standalone dewatering unit or in conjunction with an external doctor blade, wipe, or air knife for removing surface water from the roll.
In a second embodiment of the invention, an air knife is mounted in the inner volume of the roll to blow high velocity air against the inner roll surface. The air knife may be any type of non-contact air delivery system, and may be formed by a series of air knives. It may also be combined with an external doctor blade, wipe, or air knife for removing surface water from the roll.
In a third embodiment of the invention, one or more stationary air foils are positioned inside the suction roll shell to create a positive pressure region (defined herein as a pressure xe2x80x9cwedgexe2x80x9d) which exerts an outward force on the through-holes, thereby expelling residual water. It may also be combined with an external doctor blade, wipe, or air knife for removing surface water from the roll. In addition, low pressure air may be introduced ahead of the air foil(s) to increase the outward force generated by the foils.
In a fourth embodiment of the invention, the leading surface of a blowbox is aerodynamically shaped to act as a stationary foil, with outward forces being generated by both the foil and the blowbox. Low pressure air may be introduced ahead of the shaped surface. It may also be combined with an external doctor blade wipe, or air knife for removing surface water from the roll.
Another feature of the invention is a structure for selectively loading the blowbox against the inner surface of the suction roll. Thus, the blowbox can be positioned against the inner roll surface only when needed.
Other objects, features, and advantages of the present invention will be explained in the following detailed description of the invention having reference to the appended drawings.